Possible Evidence for Shear-driven Kelvin–Helmholtz Instability along the Boundary of Fast and Slow Solar Wind in the Corona

Abstract

This paper reports the first possible evidence for the development of the Kelvin–Helmholtz (KH) instability at the border of coronal holes separating the associated fast wind from the slower wind originating from adjacent streamer regions. Based on a statistical data set of spectroscopic measurements of the UV corona acquired with the UltraViolet Coronagraph Spectrometer on board the SOlar and Heliospheric Observatory during the minimum activity of solar cycle 22, high temperature–velocity correlations are found along the fast/slow solar wind interface region and interpreted as manifestations of KH vortices formed by the roll-up of the shear flow, whose dissipation could lead to higher heating and, because of that, higher velocities. These observational results are supported by solving coupled solar wind and turbulence transport equations including a KH-driven source of turbulence along the tangential velocity discontinuity between faster and slower coronal flows: numerical analysis indicates that the correlation between the solar wind speed and temperature is large in the presence of the shear source of turbulence. These findings suggest that the KH instability may play an important role both in the plasma dynamics and in the energy deposition at the boundaries of coronal holes and equatorial streamers.

Modeling of Joint Parker Solar Probe–Metis/Solar Orbiter Observations

“…In the letter, we use a superradialexpansion turbulencedriven solar model (Telloni et al 2022a) to study the coronal plasma in the slow solar wind that the hot plasma emerges from the closed loop into the open field region by interchange reconnection, after which it expands superradially. We consider a steady flow in a one-dimensional, superradially expanding the open flux tube of the cross-sectional area A(r) = r 2 f (r) (where f (r) is a superradial expansion factor), which is inversely proportional to the magnetic field strength B(r),…”

“…Parameter s 1 denotes the fraction of turbulence energy used to heat the coronal plasma (protons). We use s 1 = 0.6, meaning that the 60% of the turbulent energy heats the coronal/solar wind (proton) plasma (Breech et al 2009;Cranmer et al 2009;Engelbrecht & Strauss 2018;Chhiber et al 2019;Adhikari et al 2021b;Telloni et al 2022a). Combining the above equations yields…”

“…The 1D steady-state transport equations for the energy in NI/slab forward-propagating modes and the corresponding energy-weighted correlation length are (Zank et al 2017;Adhikari et al 2020;Telloni et al 2022a),…”

“…Similar to Telloni et al (2022a), we use two forms of the turbulent shear source: (i) in the region between the sonic surface and Alfvén surface, where the sound speed C S is assumed to be the characteristic speed, and (ii) beyond the Alfvén surface, where the Alfvén speed is considered to be the characteristic speed. The shear source of turbulence in the region between the sonic and Alfvén surfaces can be written in the form (Telloni et al 2022a) g r g r g r…”

Observations of Cross Scale Energy Transfer in the Inner Heliosphere by Parker Solar Probe

“…Beyond this Alfvén critical region, the strong coronal magnetic field loses control of the plasma. The shears introduce Kelvin Helmholtz like dynamics, creating large scale roll ups in the solar wind [20,21]. These roll-ups push the solar wind towards isotropization at the largest scales introducing an important step in its turbulent evolution [22].…”